The fleshy, bulbous area right above a budgerigar's beak just below the eyes is called a "cere." This is where the nostrils, called "nares," live. It appears to be a ridge that connects the beak to the bird's face. Though not all birds possess a cere, varieties of parrots, doves, pigeons, raptors, and some other birds of prey do.
How a Bird's Cere Can Indicate Its Sex
Experts haven't identified a single purpose of the cere, but most think it has to do with the bird's sense of smell.
It can also be a pretty good indicator of a bird's sex. Males have a darker colored cere than the females. The issue with consistency in this indication is that lighter colored birds have a lighter colored cere, so while, in theory, cere color is a fairly good indicator of the sex of the bird, it isn't that way in all cases. This only applies to adult birds as the cere color in juveniles changes as they age.
The color can also change in the female when she is in breeding condition. When she is in season and is interested in breeding, her cere might turn brown in color, but, again, this isn't always the case. Some female's ceres don't change color at all. Another indication of sex, especially among pigeons, is the fact that a male's cere is usually larger and more pronounced than the cere on a female.
Bird watchers often use the cere as a means of identifying different types of birds in their never-ending quest to see different types of birds.
How to Officially Determine the Sex of Your Bird
In any case, if you feel it is important to find out the sex of your budgie, it is best to get a DNA blood test done. The accuracy of these blood tests is about 99%. While not completely fool-proof, they trump any other method of discovering the sex of your bird.
The test is fast and only takes a drop of blood. Surgical sexing used to be the only way to determine the sex of a bird with absolute certainty, but this method is rather invasive and can be risky.
How A Bird's Cere Can Indicate Illness
Another interesting feature about the cere is the fact that its condition is a telltale sign of illness. When examining a bird, avian veterinarians often first look at the cere. If there are any signs of irritation, swelling, or inflammation, the vet is likely to look for possible respiratory issues.
Other problems that can indicate illness are a chapped cere as well as what appears to be a runny nose. Chronic respiratory illnesses and infections may cause the cere to sweep and become impacted with pus from the infection causing distortion of the cere. These plugs of pus, called rhinoliths, are more commonly observed in African Grey parrots, but they can occur in any species of bird. It is important that the avian veterinarian determine that this appearance is the result of an infection and not a tumor, or granuloma, which is essentially an inflamed tumor. Such an infection can be corrected by putting the bird under general anesthesia and flushing out this accumulation of pus caused by the infection.
The cere is one of the more obvious parts of your bird's anatomy you can observe that indicates the bird's health. Keeping an eye on it for sweeping, dripping, or any type of discharge is a good practice.
Edited b: Patricia Sund